Multiple personality disorder (MPD) is a dissociative disorder that is increasingly being recognized in victims of severe childhood abuse. Although this condition has been known for over 200 years, little systematic information has been collected on the phenomenology and symptomatology of this unusual condition. At present there are over 7000 identified cases in the United States with new cases being discovered at a high rate as better information becomes available to practicing clinicians. We have systematically collected data on over 200 cases in treatment. Our findings indicate that MPD is a clinical syndrome with a core of dissociative and affective features that occurs in individuals suffering serious, repetitive childhood trauma (primarily physical and sexual abuse) between ages two and fourteen years. MPD is typically not diagnosed until the third or fourth decade following an average of seven years of misdiagnosis and ineffective treatment. Proper treatment appears to substantially reduce the morbidity but no controlled treatment/outcome studies have been conducted to date.